Using just these five variables, we can write the three most important kinematics equations. An important
note: The following equations are valid ONLY when acceleration is constant. We repeat: ONLY WHEN
ACCELERATION IS CONSTANT . Which is most of the time.^2
We call these equations the “star equations.” You don’t need to call them the “star equations,” but just be
aware that we’ll refer to the first equation as “ ,” the second as “ ,” and the third as “ ” throughout
this chapter.
These are the only equations you really need to memorize for kinematics problems.
Constant-Acceleration Kinematics Problem-Solving
Step 1 : Write out all five variables in a table. Fill in the known values, and put a “?” next to the unknown
values.
Step 2 : Count how many known values you have. If you have three or more, move on to Step 3 . If you
don’t, find another way to solve the problem (or to get another known variable).
Step 3 : Choose the “star equation” that contains all three of your known variables. Plug in the known
values, and solve.
Step 4 : Glory in your mastery of physics. Feel proud. Put correct units on your answer .
Be sure that you have committed these steps to memory. Now, let’s put them into action.
A rocket-propelled car begins at rest and accelerates at a constant rate up to a velocity of 120 m/s. If it
takes 6 s for the car to accelerate from rest to 60 m/s, how long does it take for the car to reach 120
m/s, and how far does it travel in total?Before we solve this problem—or any problem, for that matter—we should think about the information it